I drive a 1993 Civic LX with 117k miles. I usually go around 300 miles on a tank, but about a month ago that suddenly dropped to more like 250. I haven't changed my driving habits or the fuel grade, the CEL hasn't come on, and the only work that's been done on the car recently was an oil change around the same time as the mileage drop, though I imagine that's just coincidental.

I had a full tune-up last August and have had no other problems since. My regular mechanic said to ask a dealer, but I'm a student and would rather not pay for a dealer's diagnosis if I can avoid it (or I'd at least like to have some idea of what to expect if I do). I'm not so concerned about the decreased fuel economy--I'd expect that as the car gets older--as I am about the possibility that the sudden onset is indicative of a bigger problem.

Any ideas?

 

Is your CEL (check engine light) on? Does the light turn on then off when you start the car?

 

Have your mechanic check the spark timing. If it is way off it could indicate a worn timing belt, eventual failure of the belt is likely to cause sudden severe engine damage. How old is your timing belt, both in years and in miles?

 

itinerant: Nope, the CEL does not turn on at all.

mk378: The timing belt was replaced at about 85k--30k miles and 4 years ago.

 

So even when you start the car, or have the key in the "on" position it doesn't light up? It should, if it doesn't then your bulb is burnt out and not indicating that you actually are throwing a code.

 

What precisely was done and replaced during your recent "full" tune up?

 

If you mileage has dropped dramaticly in a short span of time there is one very probable answer. The thermostat to you engines cooling system may have puller through into the engine itself, or simply be in need of a replacement. With no thermostat the coolant is constantly circulating the engine. It is supposed to only do this at a higher temperature but if the engine is constantly cold it will be running a rich mixture of fuel, causing the mileage to be extremely poor. This thermostat should not be an expensive fix and i would suggest looking into the situation as I have seen it before.
____________________
Ignition Coil Ground Strap

 

Sorry, should have answered more carefully. It does come on for a second with the others, but not other than that.

RonJ, I don't remember precisely what was done and replaced and can't find the paperwork right now. I was getting ready to drive cross-country, so I got the most thorough option my mechanic at the time had.

BANKSBaker: Thanks, I'll look into that possibility.

 

When your gas mileage drops, the first thing to check is the ignition system. Start by inspecting the plugs, distributor cap, and rotor for corrosion and wear. Consider also checking the resistance of the plug wires with a multimeter.

 

One thing you can do is take the car to Autozone and have them use their OBD scanner to see if there are any CEL codes stored in memory. They do it for free. Your CEL bulb is burned out or has been removed, so it could be throwing codes and you'd never know. I'd recommend replacing the bulb while you're at it. Its not too hard. From there, you can start troubleshooting your ignition system and beyond.